Mating systems: a simple
classification
1 male
> 1 male
1 female
>1 female
monogamy (often
cooperative);
solitary
polyandry, serial or
simultaneous
'harem' polygyny
multi-male polygyny;
promiscuity (leks);
polygynandry (coop.
laying)
Monogamy in Mammals
• Monogamy is rare in mammals
• Why? Females put in most of the parental
investment
• Exception: when male parental care can
make a difference
• Enforcement:
– Mate assistance
– Female-enforced monogamy
Monogamy in Birds
• In birds, about 90% of species display
monogamy
• Why? Because in many cases, males can be
effective parents (mate assistance)
– Male advantage: caring for one clutch gives
him higher success than deserting
– Female advantage: male care improves her
young’s chances of surviving
Social vs. Genetic mating systems:
Extra-Pair Copulations (EPC’s)
• Even “monogamous” birds may mate with
other individuals -- in fact, many do
• Benefits to male: increased reproductive
success
• Costs to male: time and energy to search for
other mates, possibility that female might
also engage in EPC
Why would females engage in EPC?
•
•
•
•
•
•
Better sperm
Good genes
Different genes
Material benefits
Access to territories
Male parental care
Polyandry
• Each male only mates with one female, but
a female will mate with multiple males
• Rare, but it happens
• Sex role reversal polyandry occurs when
females can easily lay more eggs, but at
least one parent is needed to raise clutch
• examples: Spotted sandpipers, jacanas
Polyandry II
• Polyandry can occur when more than 2
parents are needed to raise clutch or litter
• Example: Saddle-backed tamarins. Multiple
large offspring that grow quickly. Normally
cooperative breeders, w/ prior offspring. For a
new group, there are no existing offspring. Then,
a male will tolerate or even invite another male to
join group, mate with female and help to raise the
offspring. Once the offspring are established,
however, one of the males gets kicked out.
Polygyny
• Each female mates with only one male, but a
male with mate with multiple females
• Female defense (harem) polygyny. Females
occur in groups (Why? We’ll see later)
• Resource defense polygyny. Resources critical
to female RS must be clustered or otherwise
easy to defend. Some females would rather be
a second female on a good territory than a
monogamous female on a mediocre territory.
Polygyny continued
• Scramble competition polygyny. When all
else fails…. If neither resources nor females
are defendible, and females are not
predictable, then just search for females as
fast as possible.
• Lek polygyny. Neither females nor resources
are defendable, but females are predictable:
– Hotspot model: predictable in space
– Hotshot model: predictable preference for male
– Cluster model: prefer males in clusters
Predictions of Lek Models
• Hot-spot model
– 1. If remove males from lek, new males should
move into the same area
– 2. Males in all leks should have the same
average reproductive success
– 3. Female ranges should not change when
males are removed
• Hotshot model
– 1. If move alpha male to a nearby location, the
lek should follow
– 2. If remove alpha male the lek will disperse
Predictions of lek models (cont.)
• Female preference
– 1. Females disproportionately choose larger
leks
– 2. Females prefer “safe” spots for lek formation
– 3. Females may prefer prominent locations
Polygamy
• Extra-pair copulations, although individuals
may still have one “main” mate
• Sequential polygamy
– Females may deposit eggs in nests of several males
and males may accept eggs from multiple females.
At any one time, male is ‘mated’ with only one
female. Ex: many fish
• Polygynandry (Ex: rheas, tinamous)
– Females move about in groups, laying eggs in one
male’s nest at a time. Male is ‘polygynous’.